To transmit data by a wireless communications system, digital modulation techniques are involved to achieve high spectral efficiency and spectrum utilization. Amplifiers in base stations of a wireless communication system must be more linear as compared with counterpart amplifiers used for an analog transmission system. In the USA, the Federal Communications Commission, FCC, mandates certain limits for spectral regeneration. These limits are specified to be met with respect to certain requirements, such as, power used, the number of users per channel, and various other requirements. Not only must an amplifier satisfy the FCC requirements, an amplifier for wireless data communications must be linear over an extended range of input and output power.
Amplifiers are grouped by their operating characteristics into various classes. A Class A amplifier output device always conducts appreciable current corresponding with an input signal, and operates with good linearity, but loses efficiency due to high heat losses. Additionally, power dissipation occurs even without input power being supplied during dormant transmission moments. Such an amplifier is suitable for digital systems, but its use for data transmission is limited by its unacceptable low efficiency. A Class AB amplifier is turned on for a duration equal to or more than 180 degrees of its input signal cycle, and due to its short cycle time, attains a higher efficiency at a sacrifice of precise control of its linearity. Such an amplifier at a typical efficiency of 25% is suitable for use with a QPSK, Q-phase-shift keying, and a digital transmission system such as CDMA, code division multiple access transmission system.
According to known techniques, amplifier linearity is indicated by a graph depicting intermodulation power versus the output power of the amplifier. The intermodulation power results from distortion of the signal transferred by the amplifier. For a Class A amplifier the intermodulation power due to distortion directly increases with the power output of the amplifier. When graphed, such intermodulation power versus the power output is linear, and is depicted by an inclined, straight line, more or less. For a Class AB amplifier, a graph, indicating intermodulation power due to distortion versus output power, depicts a graphical line that visually reminiscent of an arc-tangent graphical line, with a central portion of the graphical line, in a narrow range, that is linear. A Class AB amplifier exhibits a desired linearity characteristic over the narrow range, which then degrades with both increased power and decreased power outside of the narrow range. Over an extended range of 20 dB, a variation in power of even a few dB is unacceptable. A problem to be solved is to provide an amplifier that has an optimum linearity over an extended range of output power, while retaining a high efficiency, and low intermodulation distortion. While the prior application to Vassilakis et al. has clear improvements in the linearity of a class AB amplifier by suitable adjustment of the bias to optimize linear performance accounting for temperature in input power, there is a need to optimize linearity for high power amplifiers (HPAs) over a prescribed frequency range.